Electronic equipment associated with modern telephony and data transmission installations is often fitted into a telephony apparatus cabinet which screens electromagnetic fields. The intention with this is, inter alia, to screen the equipment in the cabinet from exterior electro magneticfields but also to screen the surroundings from undesirable elctromagnetic fields generated by the equipment in the cabinet. Since the transmission lines can function as antennae both outside and inside the cabinet, radio frequency interference is, however transferred from the outside of the cabinet to its inside and vice versa via these lines. The effects of this interference decreases if the transmission lines are coupled in series with filters which attenuate the undesired interference but permit transmission of the desired useful signals.
It is known to utilise lowpass filters which contain coils for this purpose. Such filters are however, burdened with the disadvantage that their attenuating effect decreases for high frequencies, due to the coils for high frequency signals no longer functioning as inductive elements but rather as capacitive elements. Filters built up from such loss-free discrete components furthermore only provide attenuation through reflection, the size of the attenuation thus being heavily dependent on the impedance of the equipment connected to the filter and the impedance of the transmission lines.
There is described in SE 716525-3 a loss-impaired high frequency filter, which consists of a conductor surrounded by a ferrite tube, which has been coated with layers of different materials and where the outermost layer constitutes the earth terminal. The filter functions as a loss-impaired line for signals with a frequency exceeding some tens of MHz and therefore has a too high boundary frequency for satisfactorily suppressing interference in the lower part of the radio frequency spectrum.